engleski

MORE SMALL PREDATORS – INDICATING EFFECTIVE FISHING REGULATIONS OR ENHANCED PREDATOR RELEASE? A CASE STUDY IN THE MPA KORNATI NATIONAL PARK, CROATIA

Trammel-net fish surveys in the Croatian Adriatic between 1993 and 2009 identified increases in abundance of four meso-predator species, hypothesized to be indicators of positive effects from fisheries regulations implemented within those years. An alternative explanation is that ongoing harvest of large predatory species continues to release these populations of indicators. We tested these two hypotheses by comparing fish population densities inside and outside MPAs and no-take areas. If fishing restrictions are responsible for increase in indicator species then 1. densities of indicators should be uniformly higher in more fisheries- restricted zones. If fishing restrictions increase top predators, which then reduce the populations of meso-predators, then 2. densities of predators should increase, and their potential prey decrease, in response to fisheries restrictions. We performed trammel-net surveys at four sites, three within and one outside the Kornati National Park, Croatia in 2009, 2013, 2014 and 2015. We matched the trammel specifications, including habitat and depth, of the 1993-2009 survey. The four indicator species reached highest abundances at different locations. Symphodus tinca peaked within the no-take zone, Pagellus erythrinus at a less protected site within the MPA, and Mullus surmuletus and Scorpaena porcus peaked outside the MPA with no significant differences across MPA sites. This contradicts hypothesis 1. Hypothesis 2 can be addressed by comparing Scorpaena species, which are trophically connected. Two Scorpeana species coinhabited the site with Scorpaena porcus. The larger S. scrofa significantly increased in abundance from 2009 to 2015 within the MPA with significantly higher abundances in the no-take zone for all year/site combinations. The smaller S. notata showed no significant changes across MPA sites and displayed negative trends within the no-take zone. In less protected MPA sites, S. porcus and S. notata displayed opposite abundance trends: positive for the larger S. porcus and negative for the smaller S. notata. These results support hypothesis 2. We conclude that increases in meso-predators is not evidence for the general success of fishing regulations, but of relatively higher vulnerability of large predators to fishing, and subsequent predator release. This work was supported by the Croatian National Science Foundation under the project COREBIO (3107).

Marine protected areas, mesopradtor release, Scorpaena, trammel net survey, fisheries restrictions

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